The Effect of CO 2 Enrichment on Root Nodule Development and Symbiotic N 2 Reduction in Pisum sativum L.

Abstract

A very significant increase in N2(C2H2) reduction by Visum sativum L. infected with Rhizobium leguminosarum occurred when plants were grown in the light with 6 hr of CO2 enrichment (0.00120 atm). Plants grown for 4 wk under 0.00120 atm CO2 showed significant increases over control plants at 0.00032 atm CO2 in plant dry weight, N content, root nodule mass, number of nodules, and mean nodule dry weight. Acetylene-reduction assays, however, revealed no reproducible increase in nitrogenase activity/mg nodule in plants subjected to long-term CO2 enrichment. Both control and CO2-enriched plants optimized the sink/source ratio between the mass of nodules and the extranodular plant mass. The optimum ratio for N2 reduction by 4-week-old peas was 0.05. Long-term CO2 enrichment did not promote root nodule formation to a greater degree than total plant development, and increases in N content were directly proportional to increases in nodule mass. Morphological data revealed significantly greater deposits of starch in root nodules of plants grown under CO2-enriched conditions. The results are interpreted as showing that short-term increases in CO2 levels promote N2 reduction by affecting root nodule functioning, whereas long-term CO2 enrichment promotes N2 reduction by increasing total plant and root nodule development.